The invention relates to a drive unit which drives a medical pump used in a circulating system of a human body such as artificial heart or a balloon pump within the aorta, for example, in particular, to a drive unit for a medical pump which alternately delivers a positive and a negative pressure.
Considering a balloon pump disposed within the aorta, it is inflated in response to a positive pressure and deflated in response to a negative pressure applied thereto. Accordingly, a medical pump of the kind described performs a pumping operation in response to a repeated application of alternating positive and negative pressures. Such positive or negative pressure is applied to the pump from a given drive unit in terms of a fluid such as air or gas. An example of a drive unit for such medical pump is disclosed in U.S. Pat. No. 4,648,385.
It will be noted that to meet with the need of a varying size of the physical body of a user, an intra-aortal balloon pump is prepared in a variety of capacities. In a drive of the balloon pump, a required minimum amount of fluid, which depends on the capacity of the balloon pump being driven, should be supplied to the latter when it is to be expanded. It is preferred that the balloon be inflated and deflated at a high rate.
In an arrangement disclosed in U.S. Pat. No. 4,648,385, a fluid isolator is divided by a diaphragm into a primary and a secondary side, with an air pressure applied to the primary side while the secondary side is filled with helium gas to drive a balloon pump connected to the secondary side.
To accommodate for a change in the capacity of the balloon pump, the primary side of the fluid isolator is provided with a control member which controls the extent of movement of the diaphragm, so that the stroke of diaphragm is adjustable by adjusting the position of the control member. However, with this construction, when the balloon pump is changed, the stroke of a fresh balloon pump must be adjusted again depending on its capacity. Another factor which requires a consideration relates to the length of a hose which interconnects the balloon pump with its drive unit. When the length of the hose varies, the relationship between the capacity of the balloon pump and the stroke of the diaphragm also changes. Accordingly, the length of the hose cannot be changed at will. In addition, when the diaphragm abuts against the control member, the magnitude of a negative pressure applied to the balloon pump becomes saturated and is limited to a small value. Accordingly, when a smaller stroke is chosen, the resulting negative pressure will be reduced in magnitude, resulting in a retarded rate of deflation of the balloon pump.